Once installed, scenarios appear in the Scenarios menu, in alphabetical order.
Driver sessions, however, appear in the Driver menu, listed under the layout on which they run.
The difference between the two is subtle, but in simple terms:
Scenarios are written in a programming language called GameScript or TrainzScript (both names are used for the same thing). Scenarios can't be changed as they are run, but they can offer various choices and random events if the programmer designed them to do so.
Driver sessions are constructed using a graphical interface and involve Driver Commands and Rules. They can be changed as they are run.
HTH, John
